The use of a cantilever structure for supporting the drive shaft in a wobble plate type compressor is well known. For example, this structure is disclosed in U.S. Pat. Nos. 3,552,886 and 3,712,759.
FIG. 1 shows a conventional refrigerant compressor for use, for example, in an automotive air conditioning system. Wobble plate type compressor 1 has a conventional cantilever structure and includes cylindrical compressor housing 2 with front end plate 3 and a rear end plate at opposite ends thereof. The rear end plate is in the form of cylindrical head 4. Cylinder block 21 is located within compressor housing 2 and crank chamber 22 is formed between the interior surface of compressor housing 2, cylinder block 21, and the interior surface of front end plate 3. Valve plate 5 covers the combined exterior surfaces of compressor housing 2 and cylinder block 21, and cylinder head 4 is attached to compressor housing 2 via bolt 41 extending through valve plate 5. Front end plate 3 includes opening 31 through a central portion thereof and through which drive shaft 6 extends into crank chamber 22.
Drive shaft 6 is rotatably supported within opening 31 of front end plate 3 by radial needle bearing 7. Wedge-shaped cam rotor 8 is fixedly coupled to the end of drive shaft 6 within crank chamber 22. Cam rotor 8 is also supported on the interior surface of front end plate 3 by thrust needle bearing 9. Drive shaft 6 and cam rotor 8 rotate in unison.
Wobble plate 10 is annular and is provided with bevel gear 101 at its central portion. Wobble plate 10 is disposed on inclined surface 81 of cam rotor 8 and is supported by thrust needle bearing 16 therebetween. Supporting member 11 includes shank portion 112 disposed within central bore 211 of cylinder block 21, and bevel gear 111 which engages bevel gear 101 of wobble plate 10. Shank portion 112 includes hollow portion 113. Support member 11 nutatably supports wobble plate 10 with spherical element 12 (e.g., a steel ball) disposed between bevel gear 101 and bevel gear 111. A key is located between cylinder block 21 and supporting member 11 to prevent rotational motion of supporting member 11. Adjusting screw 17 is disposed within central bore 211 adjacent the end of shank portion 112. Coil spring 13 is disposed within hollow portion 113 and urges supporting member 11 toward wobble plate 10. The engagement of bevel gear 111 with bevel gear 101 prevents the rotation of wobble plate 10.
A plurality of cylinders 212 are uniformly spaced around the periphery of cylinder block 21. Pistons 14 are slidably fitted within each cylinder 212. Connecting rods 15 connect each piston 14 to the periphery of wobble plate 10 via ball joints. Discharge chamber 42 is centrally formed within cylinder head 4, and suction chamber 43 has an annular shape and is located within the periphery of cylinder head 4 around discharge chamber 42. Suction holes 51 are formed through valve plate 5 to link suction chamber 43 with each cylinder 212 and discharge holes 52 are formed through valve plate 5 to link each cylinder 212 with discharge chamber 42.
A driving source rotates drive shaft 6 and cam rotor 8 via electromagnetic clutch 18 mounted on tubular extension 35 of front end plate 3. Wobble plate 10 nutates without rotating in accordance with the rotational movement of cam rotor 8, and each piston 14 reciprocates within cylinders 212. The recoil strength of coil spring 13 may be adjusted by rotating adjusting screw 17 to securely maintain the relative axial spacing between thrust bearing 9, cam rotor 8, wobble plate 10, bevel gear 101, spherical element 12, and supporting member 11. However, the relevant spacing may change when compressor 1 is operated due to dimensional error in the machining of the elements and due to changing temperature conditions within crank chamber 22.
Wobble plate type compressor 1 is normally used as a refrigerant compressor in an automotive air conditioning system and should be sufficiently durable under normal operating conditions which include periods of operation under severe conditions. However, under severe operating conditions, such as driving for a long period of time in high temperatures, sometimes the driving parts of the compressor may fail to operate as desired, decreasing the durability of the compressor and causing it to malfunction. It has been determined that compressor malfunction is caused by fragmentation of bits of the exterior surface of drive shaft 6 where it contacts the interior surface of radial needle bearing 7. The fragments damage the other driving parts of the compressor, thereby causing it to malfunction.
FIG. 2 is developmental view showing the exterior surface of drive shaft 6 within radial bearing 7. (The cylindrical surface has been "unwrapped" and laid flat.) Drive shaft 6 rotates around the center of radial bearing 7 as it rotates or spins around its own longitudinal axis so that the contact surface of drive shaft 6 with radial bearing 7 does not vary. Strong contact and thus fragmentation occurs at area A. Area B indicates additional locations where contact occurs between drive shaft 6 and radial bearing 7. The contact at area B is not as strong so it is not damaged, but area B has a lustered or shined surface due to the contact. It can be seen that the exterior surface of drive shaft 6 does not uniformly and fully contact the interior surface of radial bearing 7. Fragmentation results from this non-uniform contact between the exterior surface of drive shaft 6 and the interior surface of radial bearing 7.
FIG. 3 shows the forces acting on cam rotor 8 and drive shaft 6 during operation of the compressor. The external forces acting on cam rotor 8 include a gross gas compression force F.sub.1 acting axially at point A due to the compression of each piston 14. Point A is located near the connection of connecting rod 15 with wobble plate 10 via the ball joint. The gross gas compression force acts when each piston is at its top dead center point, which occurs when the thicker part of cam rotor 8 is adjacent each piston 14. The gross gas compression force acts on inclined surface 81 of cam rotor 8 and therefore includes radial component F.sub.3. Additionally, axially urging force F.sub.2 acts on cam rotor 8 at a central location. The axially urging force is created due to the recoil strength of coil spring 13 acting cam rotor 8 via intermediate elements. The urging force also acts on inclined surface 81 of cam rotor 8 and therefore includes radial component F.sub.4.
Axial reaction force F.sub.5 is created at the contact point, point B, between cam rotor 8 and thrust bearing 9 and balances the axial forces F.sub.1 and F.sub.2. However, no reaction force is available to balance the combined force provided by the radial component forces F.sub.3 and F.sub.4 and thus, the radial component forces create a torque causing cam rotor 8 to shift around point B within the plane of FIG. 3. As a result, cam rotor 8 is separated from thrust bearing 9 at the side adjacent each piston 14 at its bottom dead center point which occurs when the thinner part of cam rotor 8 is adjacent each piston 14. Therefore, the rotational axis of drive shaft 6 is inclined with respect to the longitudinal axis of radial bearing 7, and contact occurs between drive shaft 6 and radial bearing 7 and points C and D. The angle of inclination .theta. between drive shaft 6 and radial bearing 7 depends upon the axial length of radial bearing 7 and the clearance in the radial direction between the interior surface of radial bearing 7 and the exterior surface of drive shaft 6.
Radial reaction forces F.sub.6 and F.sub.7 act on drive shaft 6 from radial bearing 7 in opposite directions at points C and D respectively. Since there is no movement of drive shaft 6 in the radial direction during operation, these forces balance the radial component forces F.sub.3 and F.sub.4 as follows: EQU F.sub.3 +F.sub.4 =F.sub.6 -F.sub.7
Since after cam rotor 8 contacts thrust bearing 9 there is no further rotation around point B, the moment around point B is represented by the following equation: EQU F.sub.3 l.sub.1 +F.sub.4 l.sub.2 +F.sub.6 l.sub.3 -F.sub.1 (r.sub.2 -r.sub.1)-F.sub.2 r.sub.2 -F.sub.7 l.sub.4 =0
where l.sub.1, l.sub.2, l.sub.3, and l.sub.4 are displacements measured in the axial direction and r.sub.1 and r.sub.2 are displacements measured in the radial direction between each force vector and point B. Each addend is the magnitude of the cross product of the two vectors. However, only one non-zero component remains after the cross product since the force and displacement vectors are perpendicular. F.sub.5 is not represented since it acts at point B.
The magnitude of radial reaction forces F.sub.6 and F.sub.7 is dependent upon the angle of inclination .theta., which is itself dependent upon the axial component of the gross gas pressure. The inclination angle .theta. is predetermined to be within a range between 0 and 0.04 degrees when a standard clearance is provided between drive shaft 6 and radial bearing 7. Therefore, the operation of the compressor under a high thermal load causes fragmentation of drive shaft 6 due to the magnitude of the radial reaction forces which create non-uniform contact with radial bearing 7.